Computer mice are a type of computer input device used to input data and control cursor movement on a computer screen. The mouse includes a roller ball mechanism protruding from the base of the housing. A user grasps the housing of the mouse to roll the ball over a flat surface, from which changes in positional data can be determined. The mouse also includes at least one button on the top of the housing which the user can depress to activate various functions. A number of companies, such as Microsoft Corporation, Logitech Inc., Assmann, and Sicos, manufacture mice which are readily available and in wide use.
Symptoms of repetitive strain injuries in the neck, shoulder, forearm, and wrist have been related to the increased use of the computer mouse. Repetitive strain injury (RSI), also known as repetitive motion disorder or cumulative trauma disorder, is an umbrella term which covers a number of similar conditions arising from overuse of the joints or soft tissues of the upper extremities. Factors leading to upper extremity RSIs are stressful, highly forceful, and/or highly repetitive arm and hand movements and positions, including ordinary movements such as gripping and reaching. These movements may become hazardous if they are repeated forcefully or awkwardly without a sufficient rest to allow recovery.
In case histories, specific musculoskeletal diseases have been linked to the mouse. Carpal tunnel syndrome, finger extensor tendinitis, and neck/shoulder myofascial pain are some of the diagnoses referred to in the reports from physical therapists and physicians in occupational health care. Hagberg, M., et al, "Symptoms and disorders related to keyboard and computer mouse use," Handout, International Conference on Occupational Disorders of the Upper Extremity, ver. 1.0, Oct. 6, 1994.
Carpal tunnel syndrome (CTS) is the most frequently encountered peripheral compression neuropathy. The carpal tunnel is bordered posteriorly by the carpal bones of the wrist and anteriorly by the inelastic flexor retinaculum, also known as the transverse carpal ligament. Ten structures traverse the carpal tunnel: four tendons of the flexor digitorum superficialis (FDS), four tendons of the flexor digitorum profundus (FDP), the tendon of the flexor pollicis longus, and the median nerve. CTS is usually gradual in onset, sometimes presenting first in a single finger and later spreading to the rest of the distribution of the median nerve.
CTS has classically been considered a nerve compression injury. Although most cases of CTS are idiopathic, a wide variety of conditions may predispose a person to CTS. A rise in carpal tunnel pressure is believed to be the direct cause of median nerve compression in CTS. Non-specific synovial proliferation within the carpal tunnel is the most common cause. It is known to be aggravated by extrinsic dynamic factors, such as activities requiring prolonged wrist flexion or repeated wrist or finger flexion and power grip. Movement of the intrinsic lumbrical muscles into the carpal tunnel during finger flexion may also affect the carpal tunnel pressure. Armstrong, T. J. et al., "Some Biomechanical Aspects of the Carpal Tunnel," 1978; Ranny, D. A. et al., "Lumbrical Function: Interaction of lumbrical contraction with the elasticity of the extrinsic finger muscles and its effect on metacarpophalangeal equilibrium," The Journal of Hand Surgery, Vol. 12A, No. 4, Jul., 1987, pp. 566-574. Furthermore, due to its interposed position between the anterior row of tendons and the transverse carpal ligament, the median nerve is subject to shearing and compression as flexor tendons are displaced. Recent research indicates that the flexor tendons consistently translocate at two to three times the rate of the median nerve and the median nerve experiences proximally directed, frictional shear forces exerted by the adjacent tendons. Szabo, R., et al, "Median Nerve Displacement Through the Carpal Canal," The Journal of Hand Surgery, Vol. 19A, No., 6, November 1994, pp. 901-906.
Many of the prior art mouse designs require or encourage the user to apply primarily a finger tip pinch to operate the mouse button. The finger tip applies force against the mouse button in a finely controlled manner. With this action, the finger flexes at all three joints and contacts the mouse button only at the finger tip. In an active pinch action, the FDP tendon carries most of the strength. Chao, E. Y., "Three-Dimensional Force Analysis of Finger Joints In Selected Isometric Hand Functions," 1975. Also, the larger and more rounded contour with a tilted or sloped palm support may have a tendency of "locking" the hand into a rigid holding posture, which does not allow easy shifting or movement away from the suggested hand posture. Maintaining the hand in a single posture increases the static loading of the muscles. See, for example, the prior art mice illustrated in FIGS. 23A, 23B, 24A, 24B, 25A, and 25B.